1. Field of the Invention
The present invention relates to a four-cycle engine for a vehicle, in which a cylinder bore is disposed in such a manner that the axial line of the cylinder bore extends substantially in the horizontal direction. Furthermore, a cam shaft is disposed between an intake valve and an exhaust valve which have operational axial lines crossing each other, forming an approximately V-shape.
2. Background Art
In a prior art four-cycle engine including an intake valve and an exhaust valve having operational axial lines crossing each other in an approximately V-shape, a diameter of the intake valve is larger than that of the exhaust valve. Accordingly, in order to make the squish area of the combustion chamber on the intake side and that on the exhaust side equal to each other, as disclosed in Japanese Patent Publication No. Hei 1-14406, an angle formed between the operational axial line of an exhaust valve and the axial line of the cylinder bore is set to be larger than an angle formed between the operational axial line of the intake valve and the axial line of the cylinder bore.
In the four-cycle engine for a vehicle in which the axial line of the cylinder bore extends substantially in the horizontal direction and the intake valve and the exhaust valve are disposed on the upper and lower sides of the cylinder head respectively, since a distance between the axial line of the cylinder bore and the outer end portion of the exhaust valve becomes large, both the cylinder head and the head cover protrude substantially downwardly. This results in the height of the engine from the road surface on which the vehicle is grounded not being able to be reduced. In particular, for a four-cycle engine mounted on a motorcycle of a type in which the axial line of a cylinder bore extends in the width direction of the motorcycle, since the position of the outer end portion of the exhaust valve exerts a large effect on the bank angle of the motorcycle, the mounting position of the engine to the body frame must be made relatively high.